Method and circuit for preventing transients from damaging a switching regulator

ABSTRACT

A voltage control regulator features a power switch in the form of a transistor that is normally actuated for a constant switch-on time interval during each variable period interval (T). The type of control required in this case comprises, conventionally, a monostable multivibrator, a controller, and a voltage/frequency converter. The present control system is subordinated to an emergency control circuit which, via a priority loop, intervenes in the control process as soon as the current flowing through the switching transistor threatens to become excessive. This emergency control unit thus constitutes a predictive type of control that employs knowledge of the operation of the voltage control regulator and the instantaneous values of the input and output voltages of the voltage control regulator, in order to predict the voltage increase during the period interval (T). Such an arrangement permits the emergency control unit to react very quickly.

FIELD OF INVENTION

The present invention relates to a method and device suitable forpreventing transients from hampering the operation of a switchingregulator (buck cell).

BACKGROUND OF INVENTION

Switching regulators based on the principle of the buck cell are known,for example, from Patent WO 84/00085 or EP/O 111 493. Such switchingregulators have a power switch that opens and closes according to apredetermined pattern, whereby energy is periodically fed to a choke andthrough the latter both to a capacitor and a load switched in parallelthereto.

The principle of the switching regulator is very simple and itsconstruction is sturdy. The switching regulator can, however, be damagedunder extreme operating conditions, especially its power switch can bedamaged. Such conditions occur primarily in the event of short-circuitsat the output and during turning the regulator on. In general, theconventional switching regulator reacts too slowly and is not able tohandle rapid changes, that is, so-called transients.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a simple and effectivemethod by which the conventional switching regulator can be safeguardedagainst the above-described extreme operating conditions and thusprotect against damage or even destruction during operation.

The proposed solution is characterized by the optimal functioning of theswitching regulator under all operating conditions. Deleterious shortswitching intervals are completely eliminated. In addition, theexcessive currents caused by transients, as well as long-lastingperturbations, are prevented from occurring.

There is more particularly provided in accordance with the presentinvention a method for preventing transients from damaging a switchingregulator, which includes at least a switch, a recovery diode, a choke,a capacitor, comprising the steps of providing a switch control thatactuates the switch throughout a continuous series of period intervals(T) of variable length for a time period (t) in each of such periodintervals that is virtually constant, and determining prior to thebeginning of each switch-on interval (t) whether or not the currentflowing through the choke will surpass a predetermined maximum value(i_(max)), and, in the event of such excess, holding the switch openuntil, at the end of the finally initiated switch-on interval (t),maximum current (i_(max)) is reached but not exceeded.

Further, in accordance with the present invention, a circuit arrangementis provided for preventing transients from damaging a switchingregulator which includes at least a switch, a recovery diode, a choke, acapacitor and a switch control for the switch, wherein the switchcontrol includes a monostable multivibrator whose switch-on interval (t)is inversely proportional to an input voltage (e), and avoltage/frequency converter suitable for sending switch-on signals tothe multivibrator. The switch control also includes a priority loopsuitable for releasing the greater of any one of two given voltages (u₁,u₂) to said voltage/frequency converter, a controller capable ofgenerating one of the input voltages to the priority loop by comparingprevalent actual voltage (U_(ist)) to a reference voltage (U_(soll)),and an emergency control unit suitable for generating the other of theinput voltage (u₁) to the priority loop.

For a better understanding of the present invention, reference is madeto the following description and accompanying drawings while the scopeof the invention will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by way of example in the accompanyingdrawings wherein:

FIG. 1 is a schematic circuit diagram of a buck-type regulator;

FIG. 2 is a schematic circuit diagram of a control circuit unit;

FIG. 3 is a schematic circuit diagram of an emergency control unit;

FIG. 4A is a waveform of the current in the choke following a shortcircuit;

FIG. 4B is a waveform of the output voltage of the regulator following ashort circuit;

FIG. 5 is a second waveform showing the steady state choke current; and

FIG. 6 is a further circuit diagram of a control circuit unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the circuitry of a prior art switching regulator 11 or buckcell, e.g., as known from the above-mentioned reference WO 84/00085.Switching regulator 11 includes a power switch 17, embodied as aswitching transistor, a free-wheeling diode 18, a choke or inductor 19;a capacitor 20; and a control circuit 21.

Switching regulator 11 connects by way of its input terminals a, b to avoltage source 13 which supplies an input voltage e, which althoughbeing d.c., can fluctuate rapidly and sharply. Connected to outputterminals c, d of switching regulator 11 is a load 15 represented by aresistance in ohms, beside which is shown the actual output voltageU_(ist).

Control circuit 21 delivers, through output 22, switching signals to theswitch 17, which closes for a switch-on time interval (conductioninterval) t during switching time interval T, and then opens during theremainder of the switching time T. Control circuit unit 21 receives ascontrol variables primarily the actual voltage U_(ist), a variablereference voltage U_(soll) and the input voltage e. Depending on theinput voltage e, the switch-on time intervals are adjusted until theproduct of t·e remains constant. Independently of this relationship,period intervals T are adjusted until U_(ist) becomes equal to U_(soll).Practical values of t and T are respectively 10 to 20 μs and 50 to 300μs.

FIG. 2 is a detailed circuit diagram of control circuit unit 21, whichsends, as described above, switch-on interval signals t and periodinterval signals T to switch 17. Unit 21 comprises a monostablemultivibrator 25, an error amplifier or controller 27, an emergencycontrol unit 28, a priority circuit 31 and a voltage/frequency converter33.

Input voltage e is fed as a control variable to monostable multivibrator25. Whenever a trigger signal arrives at input 26, monostablemultivibrator 25 is activated. After a time interval where t=k/e, (wherek is a constant) multivibrator 25 reverts to its resting state. Thissequence ensures that the product of e·t remains constant, a conditionthat is advantageous for the overall prior art control process. Amultivibrator of the above-mentioned type is also known, e.g., from theabove-mentioned reference WO 084/00085. Controller, i.e., erroramplifier 27, can, for example, be embodied as a differential amplifier,to which actual and reference voltages U_(ist) and U_(soll) are fed,while voltage/frequency converter 33 can be embodied as a commerciallyavailable VCO (voltage controlled oscillator).

Units 27, 33 and 25 constitute a conventional and known control circuitthat serves to compare and control output voltage U_(ist) with respectto reference value U_(soll) ; deviations between these two values areused by the voltage/frequency converter 33 to modify the length ofperiod interval T. Switch-on interval t meanwhile remains constant, andis influenced only by the input voltage e that changes only slightly ina given period interval T.

It is proposed that an emergency control circuit 28 operate inconjunction with this known control circuit. Priority circuit 31, which,conventionally, comprises two diodes and a resistor, releases via itsoutput 32 the greater of two voltages u₁ and u₂ which might arrive atits inputs 29 and 30. Voltage u₂, arriving at the output of controller27 is, as a rule, greater than u₁, a condition underlying the operationof such control circuit. Should, however, voltage U_(ist) besignificantly smaller than U_(soll), which always occurs for a shortinterval when output voltage regulator 11 is switched on or for a longerperiod during a short-circuit at output terminals c, d, then voltage u₁has priority at the output of emergency control unit 28, and governs theremaining process. Emergency control unit 28 hence prevents the controlcircuit from operating under unfavourable conditions for its powerswitch. Fed through inductor or choke 19 to emergency circuit 28 ascontrol variables are input voltage e, output voltage U_(ist) andcurrent i_(L).

FIG. 3 shows a further-refined circuitry arrangement of emergencycontrol circuit 28, which comprises an adder/subtractor 37, amultiplier/divider 40 and a comparator 43. Adder/subtractor 37 is, forexample, embodied as a differential amplifier. Input voltage e is fed tocontrol circuit 28 via input 35 while output voltage U_(ist) is fed viathe other input 36. Arriving at output 38 is a signal corresponding tothe difference (e-U) between the above-mentioned voltages. Issuing frommultiplier/divider 40, which can, for example, be a commerciallyavailable component MPY 100 supplied by the Burr-Brown company, is asignal I that is proportional to (e-U)/e and corresponds to a current.

Finally, in comparator 43, signal I is compared with the prevailingcurrent i_(L) passing through choke 19. Comparator 43, embodied forexample as a differential amplifier, releases via output 44 voltage u₂if i_(L) is greater than I.

In the embodiment described, analog emergency control unit 28continually extends, by means of its output signal u₁, the duration ofperiod interval T so as to ensure that, during the next switch-oninterval, the intensity of current i_(L) flowing through choke 19,cannot surpass a preset value. As soon as it is established that thispredetermined value will not be exceeded, emergency control unit 28yields its governing function and control of output voltage U_(ist) tothe above-mentioned control circuit (Units 27, 33, 35). Emergencycontrol unit therefore remains in continuous operation and reacts veryrapidly each time the above-mentioned predetermined value is reached.

FIG. 4a illustrates the waveform of current across choke 19, whereasFIG. 4b is a diagram of output voltage U_(ist). The x axis is time,while the y axis is the current, or voltage, respectively. In everyperiod interval T, switch 17 closes for an interval that is almost aslong as the next. During the remainder of each period interval T, switch17 is closed.

While current i_(L) increases, during switch-on interval t, choke 19 isloaded with energy. During the remainder of the period, choke 19 ispartially discharged and current i_(L) subsides. If the voltage U_(ist)at load 15 remains constant, it is reasonable to assume that the lengthof period intervals T will not fluctuate. Should, at this point, asudden short-circuit occur at load 15, output voltage U_(ist) fallsrapidly towards zero. The control circuit (Units 27, 33, 25) is thusable to shorten the duration of period interval T while current i_(L)rises rapidly and steeply in small increments. Before the permittedmaximum current i_(max) is exceeded during a given switch-on interval t,the emergency control unit intervenes at the end of the preceding periodinterval T and extends this interval T such that at the end of the nextprovisionally delayed switch-on interval t, the maximum current i_(max)is reached but not exceeded. The lengths of the subsequent periodintervals are then adjusted in such a way that at the end of eachswitch-on interval t, the maximum current i_(max) is reached, but notexceeded. Emergency control circuit 28 thus constitutes a type ofpredictive control that "predicts" in each period interval T the size ofthe maximum current i_(L) to occur in the following interval T. Shouldthe predicted current strength be excessive, emergency control unit 28intervenes to prevent the maximum allowable current from beingsurpassed.

The behavior and construction of emergency control unit 28 are describedin further detail in FIG. 5 which is a further waveform corresponding tothat of FIG. 4a. Along the x-axis are arranged in series two periodintervals T_(n-1) and T_(n) and their respective switch-on intervalst_(n-1) and t_(n). Along the y-axis are maximum currents j_(n-1) andj_(n) at the ends of their respective switch-on intervals t_(n-1), t_(n)and the minimum currents i_(n-1) and i_(n) at the ends of theirrespective period intervals T_(n-1), T_(n), or, rather at the beginningof each of the following period intervals T_(n), T_(n+i). The currentfluctuates during the switch-off interval (T-t)_(n-1) occurring betweenj_(n-1) and i_(n-1) and during the switch-on interval t_(n) betweeni_(n-1) and j_(n).

Due to the shortness of the intervals in the timeframe underconsideration input voltage e and output voltage U are to be regarded asconstants. Concerning the multivibrator 25, the product t·e is constant.It is required that j_(n) not exceed a predetermined maximum currentj_(max), i.e. j_(n) ≦j_(max).

We can conclude from FIG. 5 that

    j.sub.n =j.sub.n-1 -(j.sub.n-1 -i.sub.n-1)+(j.sub.n -i.sub.n-1).(1)

By applying the Law of mathematic Induction, ##EQU1##

Since t·e=φ (φ=const), the expression is transformed into ##EQU2##

This expression is realized by the circuit shown in FIG. 3 and indicatesthat, given the minimum current value i_(n-1) of period interval T_(n-1)and the knowledge of the Law of Induction, it can be predicted whetheror not, during the subsequent period interval T_(n), the maximum allowedcurrent will be exceeded. If it is decided that this value will beexceeded, period interval T_(n-1) is extended until ##EQU3## Thisensures that maximum current i_(Lmax) is virtually reached at the end ofthe next switch-on interval, as FIG. 4 shows.

If input voltage e is considered to be constant at its maximum valuee_(max), then expression (3) can be reduced to: ##EQU4## The aboveexpression indicates the maximum value permitted for j_(n-1), in orderto ensure that current j_(n) does not exceed the maximum allowed currenti_(max).

FIG. 6 shows a further circuit diagram of emergency control unit 28,which corresponds to expression (4). In this version of emergencycontrol unit 28, units 37 and 40 of FIG. 3 are reduced to one analogadder 50. Fed to the latter via input 51 is the variable output voltageU_(ist) and via inputs 52, 53, the constants i_(Lmax) and ##EQU5## Theoutput signal I' of analog adder 50 is, as FIG. 3 shows, compared bymeans of a comparator 43 to current i_(L). As soon as I' exceeds i_(L),voltage u₁ at output 44 surpasses voltage u₂ at output 30 of controller27, at which point emergency control unit 28 intervenes via the priorityloop 31 to govern switching regulator 11.

The described method for preventing transients from damaging switchingregulator (11) is simple and is embodied as described in the manner of apredictive controller. Such an arrangement permits swift reaction tovery short transients and operation in the presence of perturbations oflonger duration, such as short circuits that occur at the outputterminal. The switching arrangements required by the proposed method, inparticular those described in FIG. 6, are quite simple and can beconstructed from commercially-available electronic components.

While the foregoing description and drawings represent the preferredembodiments of the present invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the true spirit and scope of the presentinvention.

What is claimed is:
 1. A method for preventing transients from damaginga switching regulator, which includes at least a switch, a recoverydiode, a choke and a capacitor, comprising the steps of: providing aswitch control that actuates said switch throughout a continuous seriesof period intervals (T) of variable length for switch-on interval (t) ineach of said period intervals that is virtually constant, anddetermining prior to the beginning of each switch-on interval (t)whether or not the current flowing through the choke will surpass apredetermined maximum value (i_(max)), and in the event of such excess,holding the switch open until, at the end of the finally initiatedswitch-on interval (t), maximum current (i_(max)) is reached but notexceeded.
 2. A method for preventing transients from damaging aswitching regulator, which includes at least a switch, a recovery diode,a choke and a capacitor, comprising the steps of: providing a switchcontrol that actuates said switch throughout a continuous series ofperiod intervals (T) of variable length for switch-on interval (t) ineach of said period intervals that is virtually constant, anddetermining prior to the beginning of each switch-on interval (t)whether or not the current flowing through the choke will surpass apredetermined maximum value (i_(max)), and in the event of each excess,holding the switch open until, at the end of the finally initiatedswitch-on interval (t), maximum current (i_(max) ) is reached but notexceeded, and wherein the step for determining the anticipated exceedingof the predetermined maximum current (i_(max)) is derived from acombination of the prevalent actual values of the input voltage (e),output voltage (U_(ist)) and current (i_(L)) through said choke.
 3. Amethod in accordance with claim 2, wherein said combination comprisesthe expression (input voltage (e)--output voltage (U_(ist)))/inputvoltage (e).
 4. A method for preventing transients from damaging aswitching regulator, which includes at least a switch, a recovery diode,a choke and a capacitor, comprising the steps of: providing a switchcontrol that actuates said switch throughout a continuous series ofperiod intervals (T) of variable length for switch-on interval (t) ineach of said period intervals that is virtually constant, anddetermining prior to the beginning of each switch-on interval (t)whether or not the current flowing through the choke will surpass apredetermined maximum value (i_(max)), and in the event of each excess,holding the switch open until, at the end of the finally initiatedswitch-on interval (t), maximum current (i_(max)) is reached but notexceeded, and wherein said anticipated exceeding of a predeterminedmaximum current (i_(max)) is calculated from a combination of theprevalent actual value of output voltage (U_(ist)) and current (i_(L))through said choke.
 5. A circuit arrangement for preventing transientsfrom damaging a switching regulator which includes at least a switch, arecovery diode, a choke, a capacitor and a switch control for saidswitch, said switch control including:a monostable multivibrator whoseswitch-on interval (t) is inversely proportional to an input voltage(e); a voltage/frequency converter suitable for sending switch-onsignals to said multivibrator; a priority loop suitable for releasingthe greater of any one of two given inputs voltages (u₁, u₂) to saidvoltage/frequency converter; a controller capable of generating one ofsaid input voltages to said priority loop by comparing prevalent actualvoltage (U_(ist)) to a reference voltage (U_(soll)); and an emergencycontrol unit suitable for generating the other of said input voltages(u₁) to said priority loop.
 6. The circuitry arrangement in accordancewith claim 5, wherein said emergency control unit comprises anadder/subtractor, a multiplier/divider and a comparator, and thereby isable to generate an output signal (I) from said input voltage (e) andoutput voltage (U_(ist)), said output signal constituting, as opposed tocurrent value (i_(L)) at the output of choke, the other of said inputvoltages (u₁).
 7. The circuitry arrangement in accordance with claim 5,wherein said emergency control unit comprises an analog adder and acomparator which permit output voltage (U_(ist)) to be formed as anoutput signal (I'), which, in comparison to current value (i_(L)) at theoutput of said choke, forms the other input voltage (u₁).